The one I use is just a tall cheap mirror that you would hang on a door (I think I got it at Walmart). I just lean it against the tank.Haven’t had to use the mirror trick yet. This si going to sound so silly but do you just hang one off the side of the tank?
When the tang sees his reflection he “attacks” his reflection and mostly ignores the new fish in the tank. It has worked really well for me on several occasions.